Add slots to exceptions

[CoolCat467]

This PR adds slots to all the exceptions defined in _core/_exceptions.py

[codecov]

Codecov Report

Merging #2737 (57caad2) into master (722f1b5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2737   +/-   ##
=======================================
  Coverage   98.90%   98.90%           
=======================================
  Files         113      113           
  Lines       16686    16694    +8     
  Branches     3025     3025           
=======================================
+ Hits        16504    16512    +8     
  Misses        125      125           
  Partials       57       57           

Files Changed	Coverage Δ
trio/_core/_exceptions.py	100.00% <100.00%> (ø)

[jakkdl]

Is there a stronger case for this than #2723?

[CoolCat467]

In this one, nothing should be weak referencing exceptions

[jakkdl]

but is there much/anything to gain? There's a small cost in code readability and there's no CI check that makes usage consistent or anything

[CoolCat467]

I feel like it's best practice to use slots whenever possible, and more generally (not applicable here as much though) having slots on your classes means if people want to subclass them they can also take advantage of lower memory usage per object if they so choose. Not having slots eliminates that option for people down the road.

[A5rocks]

I'm very ambivalent about this one. It's probably fine! But I don't feel very strongly about specifically adding __slots__ to exceptions either way. Maybe someone else with strong(ish) opinions one way or another can figure this out...?

[oremanj]

A BaseException already has a __dict__, so the only effect of adding __slots__ = () is to disable weak references. Which only saves eight bytes per exception object, and could plausibly break something even though I agree it's unlikely.

I don't think this is worth the trouble.

[TeamSpen210]

Also exception objects aren't exactly created / kept around long, so it's not really that big a deal how big they are.

[jakkdl]

I'll close this one for now then